Texture mapping is the process of transferring a two-dimensional image onto a three-dimensional computer generated surface. For example, photographic pictures of gauges, dials and controls can be texture mapped onto a simple three-dimensional foundation surface to give a sophisticated rendering of an instrument panel without the need to geometrically model these details. Many automotive applications make use of this technique because it provides photo-realistic images at high frame rates in a cost-effective manner. Particular applications include, but are not limited to, concept vehicle design, product testing in a virtual environment and market evaluation studies.
Many commercial CAD systems are capable of creating and rendering texture maps. However, existing texture map techniques are only effective on planar or near planar surfaces. The mapped images tend to warp and distort when applied to free-form surface geometries common in automotive CAD applications. An example of such distortion is seen by comparing FIG. 1, a photograph of a vehicle tail lamp, with FIG. 2, a texture mapped CAD surface of the same tail lamp. The anomalies apparent in FIG. 2 can be corrected by a commercial artist, but this approach is both expensive and non-deterministic.
The warping and distortion seen in FIG. 2 can be traced to the transfer functions used by the texture mapping technique, which is similar to those used in most current texture mapping techniques. These techniques use a transfer function which relates the surface points of a three-dimensional object to locations in a two-dimensional image space. The function can be defined in either direction; that is, the object can be mapped to the image space or the image space can be mapped to the object. Typically, mapping from the image space to the object is done geometrically by projecting a two-dimensional algebraic surface, such as a plane or sphere representing the image space, onto the object. Any mismatch between the algebraic surface and the three-dimensional surface results in an image distortion (FIG. 2). Mapping from the object to image space must be procedural and can, therefore, be non-deterministic. Various procedures include node-relaxation and surface distance and direction methods, which still introduce image warping. Furthermore, to be computationally feasible these transfer functions are of low order, resulting in discontinuities in the mapped image.
Other existing methods can provide better results by splitting individual details into many small components, each relatively flat and with a separate texture map. However, this reduces flexibility and requires a significant amount of time and effort to produce a finished model. In U.S. Pat. No. 5,255,352, the problem of texture mapping two-dimensional texture images onto developable surfaces, that is, surfaces which can be made solely from flat panels, is addressed. This method is not applicable to free-form surfaces, however, and thus not of practical importance for the automotive industry or other industries desiring a more robust procedure. Another procedure utilizes a polygon fragmentation method of distortion correction for computer image generating systems, as disclosed in U.S. Pat. No. 5,319,744. This method tries to solve the distortion problem by pre-distorting the texture image. The method, however, does not take into account the distortion due to the perspective model used to capture the original texture image. Furthermore, the pre-distortion technique introduces several discontinuities into the mapped image.
Thus, a method is needed to efficiently texture map photographs of large sections of vehicle components onto the surface of a three-dimensional CAD object without distortion.